It is estimated that 73% of Americans between the ages of 65 to 74 get cataracts. A cataract is a clouding of the eye's lens that impairs a person's vision and, if left untreated, causes blindness. As a result, each year approximately 1.4 million people in the United States alone undergo cataract surgery, whereby the clouded natural crystalline lens is removed and replaced with an intraocular lens (IOL) implant.
Surgeons implant IOLs not only as a replacement for the natural crystalline lens but also to alter the optical properties of (provide vision correction to) an eye in which the natural lens remains. IOLs often include an optically clear disk-like optic of about 6 mm in diameter, and preferably at least one flexible fixation member or haptic which extends radially outward from the optic and becomes affixed in the eye to secure the lens in position. Implantation of such IOLs into the eye involves making an incision through the cornea. It is advantageous, to reduce trauma and increase the speed of healing, to have an incision size as small as possible.
The optics may be constructed of rigid biocompatible materials such as polymethyl methacrylate (PMMA) or deformable materials such as silicone polymeric materials, acrylic polymeric materials, hydrogel polymeric materials, and the like. The deformable materials allow the IOL to be rolled or folded for insertion through an injector or insertion cartridge and an incision into the eye. Once within the chamber of the eye, the IOL is expulsed from the injector and returns to its original shape. The primary deformable IOL materials are silicone and acrylic (hydrophobic and hydrophilic). Silicone IOLs are in general more pliable and can be folded into smaller tubes without unduly stressing the insertion cartridge or requiring excessive push force, which can suddenly expel the IOL from the cartridge. Acrylic lenses are inserted in much the same way as silicone IOLs, although in general using larger bore cartridges to mitigate the problems caused by the typically lower flexibility of the acrylic. Because the cartridge bore is larger, the incision is also necessarily larger, though some acrylics are implanted through micro incisions (<2 mm).
Injectors for delivering IOLs into the eye typically employ a handpiece and a removable cartridge that receives the IOL and has a hollow insertion tube or cannula through which the folded IOL is passed using a push rod. Some injectors do without the cartridge and are reusable. The IOL is stored separately and transferred to a load chamber in the injector or cartridge just prior to delivery. One particularly useful arrangement wherein the cartridge folds over an IOL is disclosed in U.S. Pat. No. 4,681,102 to Bartell. A cartridge opens to receive an IOL in a load chamber, and then folds closed and fits into an injector. A syringe-like plunger in the injector pushes the IOL from the load chamber through a tapered tube into the eye. The IOL unfolds as it emerges from the tip of the tapered tube. Another such insertion system is disclosed in Makker et al., U.S. Pat. No. 5,942,277.
The goal of achieving safer cataract surgery and reduced patient recovery time has resulted in smaller and smaller incisions, often less than 3 mm. Because the tip diameter of the injector or cartridge must be reduced accordingly, the IOL must likewise assume a smaller profile as it passes into the eye. This increases the force required to push the IOL through the small cartridge lumen, and impacts on the dynamics between the plunger and the IOL. Even with the introduction of a viscoelastic medium therebetween, the forces required to push the IOL against friction through the cartridge sometimes result in the push rod riding over the IOL, pinning one of the haptics against the inner lumen of the cartridge, or damaging to the trailing edge of the IOL.
Moreover, the IOL is deformable so the point where it is engaged may shift if the push rod does not have sufficient lateral stiffness. Likewise, the deformed shape of the IOL may result in a reaction force on the plunger with a force component in a lateral direction that displaces the push rod, causing it to bypass the IOL. Although one solution is to stiffen the push rod by increasing its cross-section, and reducing its length, these are not practical modifications as they run counter to the desire for a small and often elongated cartridge lumen.
One attempt to maintain contact between a plunger and IOL is seen in U.S. Pat. No. 5,494,484 to Feingold, which discloses a plunger having a faceted tip configured to engage the trailing edge of the IOL optic. U.S. Pat. No. 6,010,510 to Brown, et al. includes an injector having a plunger with an offset tip. The tip flexes up onto the floor of the cartridge so that there is a constant downward bias and the tip does not ride over the IOL. U.S. Pat. No. 6,558,395 to Hjertman discloses an insertion system with an elastic plunger capable of being reshaped, and having a guiding groove in the floor of the cartridge for stabilizing and centering the plunger. Also, U.S. Pat. No. 6,093,193 and U.S. Patent Publication No. 2007-0005135, both to Makker, et al., disclose push rods having soft tips.
Despite a number of advances in this field, there remains a need for a more gentle IOL insertion system that reduces the chance of a push rod or plunger from damaging, bypassing or otherwise riding over an IOL as it proceeds through small cartridge lumens.